Transport minister Humza Yousaf says the bridge was never intended to eradicate the congestion issues which face drivers, while official point to improvements in journey times in recent months.

Motorists travelling across the bridge into Edinburgh in the morning during rush hour faced an average speed of 24 mph in the eight months to May, according to figures obtained by the Conservatives through Freedom of Information. The return journey in the evening saw average speeds of 19.94mph. The figures covered the eight month period until the end of May this year. Commuters heading the other way, out of the capital, in the morning faced average speeds of 24 mph on the bridge, and return journeys in the evening of 21 mph.

Transport Scotland chiefs insist that the situation has eased in recent months since the 70mph speed limit was introduced and drivers got used to road arrangements.

But commuters heading out of Edinburgh to Fife in the evening rush hour still face speeds of 25 mph, although the morning journey into the capital sees speeds of 41pmh during April and May. Drivers heading back to the capital from Fife in the evening rush hour are still generally held back to half the speed limit at 35mph on the bridge, while the morning journey out of the capital sees speeds of 40mph.

Tory transport spokesman Jamie Green said the bridge has been more about an “SNP PR strategy.”

He said: “The Queensferry Crossing was supposed to revitalise the economies of the surrounding areas, as well as ease congestion and connect the east of Scotland.

“The disappointing reality is that most motorists using the Queensferry Crossing could be overtaken by a bicycle.

“These slow speeds throughout our road network are a source of huge frustration to motorists and are an active hindrance to economic growth.

It comes as the new bridge, which opened last September, was recognised as “project of the decade” by Ground Engineering.

Mr Yousaf dismissed the criticism. “What a miserable bunch the Conservatives are - they could probably turn a Bar Mitzvah into a wake, they’re so miserable,” he said.

“The clue is frankly in the name - it’s a replacement crossing. It was never designed, of course, to be a silver bullet to all the problems of congestion and so on and so forth that are faced in that very busy stretch. The Conservatives should actually get behind this project instead of talking down one of the best infrastructure projects of the decade.”